This invention relates generally to the measurement of heat flux, that is the measurement of the amount of heat transferred across a surface per unit area per unit time, and, more specifically, to the measurement of heat flux utilizing the optical properties of thermographic phosphors. The invention is a result of a contract with the Department of Energy (Contract No. W-7405-ENG-36).
The measurement of heat flux is important in many experimental situations, such as those where heat transfer must be limited and therefore monitored. For example, accurate measurement of heat-transfer rates is considered critical to the design improvements envisioned for high-pressure turbine engines. Improved understanding of the effects that contribute to heat load can lead to increased efficiency. Of particular interest is the heat transferred from the free-stream gas to an engine component surface. Examples include turbine blades and vanes.
Previous heat flux gauges have principally involved some form of resistance thermometer temperature sensor applied on both sides of an insulating medium as conducting surfaces. These conducting surfaces can also be made from pairs of materials in a thermocouple configuration. Leads connected to these surfaces would carry an electrical current which is proportional to the surface temperature detected by the sensor to an external instrument which would measure the temperatures of the surfaces. A typical gauge is made by depositing thin layers of an electrically and thermally conductive material onto both sides of a thin sheet of insulating material such as Mylar.sup.R or Kapton.sup.R.
Heat flux, Q, incident on an ideal gauge made in this way is given by the following equation: EQU Q=k(delta T)/L;; 10
where k is the thermal conductivity of the insulator, L is the thickness of the insulator, and delta T is the temperature difference between the two conductive surfaces. This equation assumes that the conductive surfaces are infinitely conducting and infinitely thin.
Modern embodiments of this configuration are disclosed in U.S. Pats. No. 4,779,994, 4,722,609, and 4,577,976.
U.S. Pat. No. 4,779,994 to Diller, et al. discloses a fairly conventional heat flux gauge which utilizes thin film layers applied to each side of a planar thermal resistance element, with its "cold" junctions applied to one surface and its "hot" junctions applied to the other. The use of thin films allows the deposition of a large number of junctions onto a small surface area which can be interconnected in series. Of course, these junctions are of the electrical resistance type, and require electrical connections.
U.S. Pat. No. 4,722,609 to Epstein et al. discloses a double sided, high-frequency response heat flux gauge consisting of a metal film approximately 1500 Angstroms thick applied to both sides of a thin (25 micrometer) polyimide sheet. At low frequencies, the temperature difference across the polyimide is a direct measure of the heat flux. At higher frequencies, a quasi-one-dimensional assumption is used to infer the heat flux. Numerous such gauges are arranged in a serpentine pattern and applied to the surface of a turbine blade.
Yet another thin film heat flux gauge is disclosed in U.S. Pat. No. 4,577,976 to Hayashi et al. wherein a pair of metallic thin films are attached to opposite surfaces of a heat resistive thin film. The heat flux through the heat resistive film is determined by measuring the temperature gradient therein while using the metallic thin films as resistance thermometer elements.
The pervading problem plaguing the above heat flux gauges, as well as all prior art heat flux gauges, is that they are electrically based. Thus, they all require connecting wires of some type in order to operate. This complicates their use, and severely limits their application to rotating components, as wire connections would have to be through slip rings. This severely complicates such an application, and greatly detracts from its reliability.
Connecting leads or wires of the prior art also limit the spatial resolution when multiple heat flux gauges are needed to measure the spatial distribution of heat flux. The degree of complication, because of the inherent geometry of such electrically based gauges, effectively precludes their use in measuring the spatial distribution of heat flux with acceptable resolution and areal coverage. Wiring dozens of gauges is complicated and interferes with the natural heat transfer to or from the surface under test. Connecting wires also present problems when such gauges are used in hostile environments.
The current invention solves the problems of the prior art by providing a leadless heat flux gauge that uses light instead of electrical means as its interrogating medium. The sensing elements of the gauges are thermographic phosphors, whose emission lines in the luminescence spectrum are temperature dependent. This allows accurate temperature determination when the phosphors are interrogated by ultraviolet light, and the spectral lines of the emitted light is analyzed. It also allows for a heat flux gauge requiring no electrical connections between the gauge and the associated evaluation and display equipment.
It is therefore an object of the present invention to provide apparatus for the accurate measurement of heat flux.
It is another object of the present invention to provide a heat flux gauge that does not require electrical connections.
It is still another object of the present invention to provide a heat flux gauge that will operate in a hostile environment.
It is still another object of the present invention to provide a heat flux gauge that is interrogated with light.
Additional objects, advantages and novel features of the invention will be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.